Studies are being carried out to further define the nature of functional and structural changes of the kidney during development. These studies are being carried out on newborn rats, dogs and pigs. Utilizing both in vitro and in vivo techniques a variety of determinations are being made. Some of these include organic anion and cation transport, total renal blood flow, intrarenal blood flow distribution, acid-base balance, electrolyte excretion, plasma renin and aldosterone concentration. Studies are being undertaken in animals at various ages and under a variety of conditions. Effects of modifications in nutritional status, acid-base balance and cardiovascular status are being studied.